Conventionally, a photoelectric fire detector that detects smoke due to a fire receives a sampling command every fixed period from a host device such as a receiver or relay to set a predetermined smoke detecting operation time. Also, it drives a light-emitting portion one time during this smoke detecting operation time, receives with a light-receiving portion light scattered by the smoke of a fire among the light from the light-emitting portion and outputs a smoke light reception signal, and based on this smoke light reception signal, detects a smoke detecting signal corresponding to the smoke density and transmits it to the host device. Also, in the case of having detected with the detector itself that the smoke detecting signal is greater than or equal to a predetermined fire threshold value, it transmits a fire interruption signal to the host device, and outputs a fire warning by specifying the fire detector that detected the fire with a search command from the host device.
In this kind of conventional photoelectric (analog-type) fire detector, in the case of noise that enters from the power supply line or noise passing through the air becoming mixed with the light reception signal output from the light-receiving portion, there is the problem of misjudging a fire due to the effect of the noise.
In order to solve this problem, for example, Patent Document 1 discloses a fire detector that, during a smoke detection time that is set in each fixed period, detects as a noise detection signal a zero-point light reception signal that is output from a light-receiving portion at the light emission stop timing of the light-emitting portion, and in the case of the noise detection signal being equal to or greater than a threshold value, judges there to be noise, and carries out predetermined noise removal processing.
This noise removal processing, for example, by not using for the fire judgment the smoke detection signal detected in the case of having assessed noise, but rather using for the fire judgment the smoke detection signal that was detected and held in the period prior to assessing noise, errors in fire judgment due to the influence of noise are prevented.